Recent Development of Nickel-Based Electrocatalysts for Urea Electrolysis in Alkaline Solution

Abstract

Recently, urea electrolysis has been regarded as an up-and-coming pathway for the sustainability of hydrogen fuel production according to its far lower theoretical and thermodynamic electrolytic cell potential (0.37 V) compared to water electrolysis (1.23 V) and rectification of urea-rich wastewater pollution. The new era of the "hydrogen energy economy" involving urea electrolysis can efficiently promote the development of a low-carbon future. In recent decades, numerous inexpensive and fruitful nickel-based materials (metallic Ni, Ni-alloys, oxides/hydroxides, chalcogenides, nitrides and phosphides) have been explored as potential energy saving monofunctional and bifunctional electrocatalysts for urea electrolysis in alkaline solution. In this review, we start with a discussion about the basics and fundamentals of urea electrolysis, including the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER), and then discuss the strategies for designing electrocatalysts for the UOR, HER and both reactions (bifunctional). Next, the catalytic performance, mechanisms and factors including morphology, composition and electrode/electrolyte kinetics for the ameliorated and diminished activity of the various aforementioned nickel-based electrocatalysts for urea electrolysis, including monofunctional (UOR or HER) and bifunctional (UOR and HER) types, are summarized. Lastly, the features of persisting challenges, future prospects and expectations of unravelling the bifunctional electrocatalysts for urea-based energy conversion technologies, including urea electrolysis, urea fuel cells and photoelectrochemical urea splitting, are illuminated.

Keywords: alkaline medium; electrocatalysts; nickel; urea electrolysis.

Figure 4

(a) TEM image of Ni(OH)₂-NMs; (b) comparison of LSV curves of Ni(OH)₂-NMs and Ni(OH)₂-NPs in 1 M KOH with 0.33 M urea; (c) the calculated formation energies of NiOOH from Ni(OH)₂ and adsorption energies of urea on the edge and basal plane of Ni(OH)₂; (d) TEM image of C@NiO; comparison of (e) LSV curves and (f) chronoamperometric curves of C@NiO and commercial Pt/C in 1 M KOH with 0.33 M urea. (a,b) Reprinted with permission from Ref. [28]. (c) Reprinted with permission from Ref. [30]. (d–f) Reprinted with permission from Ref. [32].

Figure 6

(a) Illustration of the preparations undertaken and (b) TEM image of Ni@C-V₂O₃/NF; (c,d) TEM images of NiS@Ni₃S₂/NiMoO₄; (e) evaluation of C_dl values for NiS@Ni₃S₂/NiMoO₄ and controlled samples; (f) the proposed UOR catalytic mechanism of NiS@Ni₃S₂/NiMoO₄; (g) TEM images and (i) TOF values of FeNi₃-MoO₂ and controlled samples. (a,b,g–i) Reprinted with permission from Refs. [41,45]. (c–f) Reprinted with permission from Ref. [42].

Figure 8

(a) Cobalt content dependence of electrical conductivity and onset potential; (b) chronoamperometric curves of Ni-M LDH in 1 M KOH and 0.33 M urea; (c) proposed reaction mechanism and (d) LSV curves of NiSn sulfide catalysts in 1 M KOH with 0.33 M urea; (e) density of states of S-doped and pristine Ni(OH)₂ and (f) comparison of peak current densities obtained in 1 M KOH and 0.33 M urea. (a) Reprinted with permission from Ref. [54]. (b–d) Reprinted with permission from Refs. [55,57]. (e,f) Reprinted with permission from Ref. [58].

Figure 11

(a) SEM and HRTEM image of h-NiS. (b) HER-LSV curves of o-Ni₉S₈ and h-NiS, reprinted with permission from Ref. [73]. (c) HRTEM images and (d) HER performance of the Ni-GF/VC catalyst, reprinted with permission from Ref. [71]. (e) TEM image and (f) LSV curves of Ni-P-Pt/NF catalysts for the HER in 1 M KOH aqueous solution with iR correction, reprinted with permission from Ref. [76].

Figure 1

Schematic illustration of urea electrolysis for H₂ generation and the sources of urea.

Figure 2

Scheme organization of the Ni-based HER, UOR and the bifunctional electrocatalysts discussed in this review.

Figure 3

(a) Illustration of the indirect oxidation mechanism for the Ni(OH)₂ catalyst and two-stage reaction mechanism diagrams for the Ni₂Fe(CN)₆ catalyst in (b) the first stage (the reaction from urea to NH₃) and (c) the second stage (the reaction from NH₃ to N₂). (a) Reprinted with permission from Ref。 [15]. (b,c) Reprinted with permission from Ref. [17].

Figure 5

(a) SEM image and (b) illustration of vertically aligned NiO nanosheets in UOR application; (c,e) illustration of the synthetic process and (d,f) SEM image of (c,d) multilayer Ni(OH)₂ in water and (e,f) single layer Ni(OH)₂ in methanol solution. (a,b) Reprinted with permission from Ref. [35]. (c–f) Reprinted with permission from Ref. [37].

Figure 7

(a) Illustration of the synthetic process and (b) CV curves of NiCo LDH samples in 1 M KOH with 0.33 M urea; comparison of (c) CV curves and (d) stability tests of α- and β-Ni(OH)₂ in 1 M KOH with 0.33 M urea; (e) illustration of the preparation of Ni vacancies in α-Ni(OH)₂; (f) DFT simulation at the Fermi level induced by Ni vacancies and (g) the calculated formation energies acquired to form active γ−NiOOH; (a,b,e–g) Reprinted with permission from Refs. [46,48]. (c,d) Reprinted with permission from Ref. [47].

Figure 9

(a) Illustration of a schematic fabrication of NiMo nanowire arrays via magnetic field-assisted growth and SEM image. (b) LSV curves and (c) corresponding Tafel plot of NiMo nanowire arrays, NiMo-65; reprinted with permission from Ref. [66]. (d) SEM and TEM images and (e) polarization curves of Ni₃N nanosheets with a scan rate of 2 mV s⁻¹; reprinted with permission from Ref. [67].

Figure 10

(a) Schematic representation. (b) HER-LSV curves of the Ni-Fe Janus nanoparticles, theoretical comprehension. (c) Ni-Fe heterojunction interface structure that has been optimized. (d) Standard free energy diagram of the HER process on the surfaces of Fe₂O₃(311) and Ni(111) in the Ni-Fe heterojunction. (e) HER polarization curves, reprinted with permission from Ref. [69], and (f) overpotentials at typical current densities of various LDHs, reprinted with permission from Ref. [70].

Figure 12

(a) Schematic illustration. (b) SEM images of the NiFe-MOF array. (c) LSV plots obtained with a NiFe-MOF, bulk NiFe-MOF, Ni-MOF and calcined NiFe-MOF for the HER at 10 mV⁻¹ in 0.1 M KOH. (d) LSV of NiFe-MOF for HER before and after chronoamperometric testing for 2000 s at 0.2 V (versus RHE) in 0.1 M KOH; the inset of (d) shows corresponding chronoamperometric profile. Reprinted with permission from Ref. [79].

Figure 13

(a) STEM images of NiSA-MoS₂/CC and the magnified images of (b) region 1 and (d) region 2; Ni atoms are represented by red crosses and Mo atoms are represented by green crosses. EELS spectra of Ni for (c) inner region 1 and (e) edge sites at region 2. (f) Atomic-level configuration of the absorption of Ni on the basal plane for both the top of the Mo sites (T) and the center of the hexagon sites (H); Mo atom substitution in the MoS₂ monolayer with Ni and the configuration of Ni absorption on the S-edge at T and A sites; and Mo atom substitution at the Mo-edge and S-edge with Ni. (g) Ni adsorption energy on MoS₂ and Mo atom substitution energy in MoS₂, as well as H* adsorbed Gibbs free energies at the basal plane, the Mo edge and the S edge either with or without Ni adsorption/substitution. Reproduced from Elsevier [81].

Figure 14

Polarization curves of NiSA-MoS₂/CC catalyst in (a) 1 M KOH and (b) 0.5 M H₂SO₄ solution with a scan rate of 5 mV s⁻¹ and (c,d) the corresponding Tafel plots. Reproduced from Elsevier [81].

Figure 15

(a) HRTEM image of the AC-Ni/NF sample and (b) calculated free energy for atomic hydrogen adsorption on Ni, Ni(OH)₂ and Ni/Ni(OH)₂. (c) Mechanism for the enhanced HER on the Ni/Ni(OH)₂ heterostructure. (d) Polarization curves of AC-Ni/NF and contrast samples at a scan rate of 2 mV s⁻¹ in 1M KOH. Reprinted with permission from Ref. [82].

Figure 16

(a) TEM image and (b) HRTEM image of Ni₃N@CQDs. (c) LSV polarization curves of Ni₃N@CQDs in comparison with a platinum (Pt) electrode, pristine Ni₃N, CQDs and a glassy carbon (GC) electrode in a 1 M KOH aqueous solution. (d) Tafel slopes. (e) Normalized HER amperometric I-t curves of Ni₃N@CQDs and Ni₃N at a constant overpotential of 77 mV (−1.1 V vs. Ag/AgCl). (f,g) Comparison of the HER Volmer reaction step and the resultant binding energies on (f) carbon-coated Ni₃N(110) and (g) pristine Ni₃N(110) surfaces. Reprinted with permission from Ref. [83].

Figure 17

(a) Simulation diagram of a two-electrode electrolytic cell. (b) Full electrolytic polarization curves of Ni, N-NiMoO₄/NF-20 in different electrolytes. (c) Full electrolytic polarization curves of various two-electrode electrolyzers and (d) amperometric I-t curve (inset is the actual two-electrode electrolyzer), reprinted with permission from Ref. [89].

Figure 18

(a) Photograph of the urea electrolyzer, (b) overall urea electrolytic LSVs in urea electrolyzers containing different electrode pairs, (c) LSVs of overall water electrolysis and urea electrolysis and (d) chronopotentiometry of NiO/Ni₂P/NF-40||NiO/Ni₂P/NF-40 at 10 mA cm⁻² for urea electrolysis. Reprinted with permission from Ref. [90].

Figure 19

(a) LSVs of water electrolysis and water-urea electrolysis, (b) LSVs, (c) chronoamperometric responses of NiFe-LDH/MWCNTs/NF||NiFeLDH/MWCNTs/NF and (d) comparison of the potentials of different catalysts during the UOR. Reprinted with permission from Ref. [91].

Figure 20

(a) Overall electrolysis in a two-electrode system in 1 M KOH with 0.33 M urea and (b) long-term stability over 50 h. The inset shows the polarization curves before and after the long-term stability test. Reprinted with permission from Ref. [92].

Figure 21

(a) LSV curves of MS-Ni₂P/Ni_0.96S/NF, (b) LSV curves in 0.1 M KOH with 0.5 M urea, (c) Tafel plots and (d) LSV curves of MS-Ni₂P/Ni_0.96S/NF at different scan rates. Reprinted with permission from Refs. [97,98].

Figure 22

(a) Schematic illustration of the preparation of Ni-S-Se/NF, (b,c) SEM images and (d) XRD patterns of Ni(OH)₂/NF and Ni-S-Se/NF, respectively. Reprinted with permission from Ref. [101].

Figure 23

(a) Calculated water adsorption energy of Ni-Se, Ni-S and Ni-S-Se systems. (b) The calculated configuration of water adsorbed on the Ni-S-Se system (H₂O in the circle). (c) Calculated adsorption free energy of H* on different sites of Ni-Se, Ni-S and Ni-S-Se systems. (d) The calculated configuration of H* adsorbed on the Ni-S-Se system (H* in the circle). (e) The DOS of sulfur’s p-orbital for Ni-S and Ni-S-Se systems; the p-band center is marked by dotted lines. (f) The DOS of Ni d-, S p- and Se p-orbitals of the Ni-S-Se system. Reprinted with permission from Ref. [101].

Figure 24

(a) Schematic illustration of the fabrication of NiCoP/CC; (b) low and (c) high-magnification SEM images; (d,e,g) TEM and (f,h) HRTEM images of the NiCoP/CC; and (i) SEM image and corresponding elemental mapping images of Ni, Co and P. Reprinted with permission from Ref. [103].

Figure 25

(a) LSV curves of an alkaline water electrolyzer and an alkaline urea electrolyzer using Ni/C-1 as a catalyst for both the HER and OER; (b) LSV curves of an alkaline urea electrolyzer using Ni/C-1 and Ni/C-0 catalysts; and (c) long-term durability tests of a urea electrolyzer. Inset: evolution of H₂ and N₂ gas. Reprinted with permission from Ref. [109].

Figure 26

(a) Schematic illustration of the urea electrolyzer using V-NiN/NF HER and UOR. (b) Polarization curves of V-Ni₃N/NF for urea and water electrolysis. (c) Long-term stability test performed at 10 mA cm⁻². Reprinted with permission from Ref. [113].